Fall Holy Days or Holidays?

Which days did Jesus, His disciples, and early Christians observe that came in the Fall? Do we have any references from them in the Old and New Testaments? Might the gap between Pentecost in the 2nd chapter of the Book of Acts and the fulfillment of the Feast of Trumpets symbolize the Christian church age? What about later Greco-Roman-Protestant holidays, Emperor Constantine, and Mithraism? …